What is a spotting scope?

A spotting scope is the tool of choice for hunters, shooters, birders, and wildlife observers. They are the terrestrial equivalent of astronomical telescopes, meant to be used for daytime observation. They are simple enough to be transported and quickly set up by a single person on a tripod or on a rifle. They can also be used by hand without mounting them. They are more powerful than ordinary binoculars but more compact than astronomical telescopes. They are reinforced to make them robust in the trail. Many models are made to be fog proof and water proof.

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How to Choose a Spotting Scope

Magnification

Magnification is used to make a target look much closer than it is. If you want to observe wildlife that must not be disturbed or targets that you cannot approach such as forest fires, dangerous animals, inaccessible sites, distant landmarks, spotting ships from shore, etc, you need a spotting scope. On a scope, magnification is marked as 20 X 80, where the first figure, “20X” is the magnification and “80” is the size of the objective lens.

Using my spotting scope at a magnification of just 20X, I can view a snake, skunk or beehive from a safe distance of 60 feet. I will be able to view them as if they were at an arm's reach. If I wanted to look squarely at a monastery built on top of a mountain without visual aids, I would have to view it closely from an elevated position, facing it. With a spotting scope, the monastery will look 20 times closer at a magnification of 20X and 30 times closer at a magnification of 30X.

Magnification that is beyond what the scope can support without compromising on the quality of the image is useless magnification. You must not buy a scope based on the advertised magnification.

Overall quality of the Spotting Scope

The overall quality of a scope determines how well it handles distance, dim light and resolves finer detail. Its precision and durability, both key components in lasting value of the scope, will be determined by its mechanical quality.

Quality cannot be quantified and, therefore, cannot be listed in standard specifications of the scope, and it’s difficult to describe. The quality of a spotting scope is affected by:

• The purity of the glass used in the lens
• ​The ultra precision of the lens grinding and polishing equipment (measured in a fraction of wavelength of green light)
• ​The alignment of the glass elements
• ​Materials used
• ​Lens coatings and their nature
• ​Optical tube construction
• ​Precision manufacturing
• ​Assembly
• ​Light baffling
• ​Fog proofing and purging
• ​Interchangeable oculars
• ​Lens positioning
• ​Eyecups
• ​Tripod mounting system
• ​Protective finish
• ​Structure and type of prism
• ​Quality and design of the eyepieces
• Quality control and rigorous testing

Some of the quality features cannot be gauged based on numbers and scientific terms. To gauge the quality of a scope, you need to look through the spotting scope. The quality features to look for include: detail resolution, crisp clear images, high contrast images, undistorted images in the whole field of view, as well as color rendering. The scope should be easy to focus smoothly and have easy controls. Those are some of the hallmarks of a quality spotting scope.

A manufacturer's past record of providing quality products is not proof or guarantee of future results. It is, however, a great indication of what you might get. In spotting scopes, you more often than not get what you pay for.

Specifications

Most of the advancement that has been made in the optical technology is in the design and making of the glass lenses. I will explain some of the commonly used technical terms.

Glass coatings

To reduce the reflection and scattering of light falling on the glass surface, glasses coating are applied to optical elements. Uncoated lens transmits less light than a properly coated lens. To fully multi-coat a lens, multiple coats are applied to all glass-to-air surfaces. Each coat affects different wavelengths of light and has a different purpose.

Types of glass

Extra-Low Dispersion (ED) and High Density (HG) Glass elements contain fluoride and Flourite (FL) components. The HG lenses focus all wavelengths to the same point when achromatic (transmit light without separating it into constituents colors) or to closely the same point when apochromatic (separates colors during transmission). The ED glass synchronizes all colors and projects them to the same point.

The above exotic glass lenses are difficult to manufacture and expensive. When used in a lens, they improve the scope's ability to show fine detail, image brightness, contrast color shades and faithfully render colors. They also reduce eye strain, reduce chromatic aberration (different colors focusing on different points after transmission through the lens), and enhance image brightness in low light conditions. They also increase the prices of scopes but excluding them will compromise the quality of the scope.

The Prism System

The prism in a scope corrects the image by making sure it is upright and correct from light to left. It also folds the light path to make the scope is smaller and portable. Without it, the scope would be longer and images would appear upside down and backward.

Porro prisms – They are used by most scopes. For a given price, they are easier to manufacture and give a superior performance. They can be either a BAK-4 or BK-7. The BAK-4 is superior and offers better performance than BK-7.

Roof Prisms – They are used when streamlining and size reduction are crucial. They are more expensive to produce but smaller in size. They also require extra phase alignment coats to perform optimally.

Clear Aperture

The aperture of a well-designed scope is the same as the diameter of its objective lens. This is the lens facing the target or subject being viewed. The lens facing your eye is the eyepiece. It determines how much light is gathered and ultimately how much light reaches the viewer's eye. 

A bigger lens will deliver more light to your eye but it is also expensive to manufacture to the right standards. It also makes the scope heavier as well. I recommend a bigger scope if viewing will most likely be happening in dim light conditions.

Focal length

The focal length of the spotting scope and the focal length of the eyepiece (ocular), determine the magnification or power of the scope. The shorter the focal length of the eyepiece, the more powerful the scope is.

Eyepiece

The eyepiece lens systems may have from two to eight or more lens elements. To focus the three primary colors to the same point, at least, three lens elements are needed. A fourth lens element is needed to correct any errors. Expensive scopes use additional lens elements to provide zoom or variable magnification and increase eye relief and virtual field of view.

As the level of magnification is increased, the exit pupil, eye relief, viewing comfort and overall clarity decrease. It takes a premium eyepiece to maintain the image's brightness and resolution at all zoom levels. Every movement should also be magnified even at high magnification, stability is therefore needed. This is why a tripod is essential if ultimate performance is required. Interchangeable eyepieces need to be expertly designed to maintain the telescope's performance. A poor eyepiece will definitely make a poor performing telescope.

Exit Pupil

The size of the light beam exiting the eyepiece (ocular) should be equal to the observer's pupil in the ideal case, for the brightest view to be achieved. It is obtained by dividing the diameter of the objective lens with the magnification of the scope. Adequate exit pupil even at the highest magnification is crucial to get the best performance throughout.

Twilight Factor

This is a mathematical factor used to explain how the scope's magnification and size affect its performance in dim light conditions. For example, a 15X60 scope and a 20X80 scope will have the same amount of light reaching the observer's eye. Other factors being constant, the scope with 80mm clear aperture will have a greater twilight factor and the greater magnification will allow it to resolve objects in great detail.

Eye relief

This is the minimum distance your eye should be positioned from the ocular for a full field view. If the eye relief is too little, viewing will be uncomfortable. If you wear eyeglasses, more eye relief is required. For example, a 10mm eye relief is okay for viewing with a naked eye, but if you wear glasses, at least, 20mm eye relief will be required.

I advise you to forget the numbers you see in most scopes, test the scope yourself. A fuzzy or distorted image is useless irrespective of all specs listed by the manufacturer.

Atmospheric conditions

We live at the bottom of an ocean of air full of suspended particles and air currents. Sandstorms, haze, fog and other environmental factors may interfere if not totally obstruct viewing. However, a better scope will always improve viewing conditions at twilight. The expectation must be kept within reality; you should not expect to view a rat that is located four miles away.

Angled or Straight-Through scope

When viewing out in the field for prolonged periods or multiple individuals are taking turns to view a subject, angled scopes are the best. The straight scopes are best at acquiring targets fast and when viewing at about eye level, such as when used on a shooting bench.

Focus Adjusters

These come in two types, the helical or collar focus adjuster that surround the scope's body, and the knob focus adjuster that protrudes above the prisms. Some scopes have a single adjuster for precise focusing while other scopes feature two adjusters for dual focusing. One adjuster is for coarse focusing and a slower knob for precise fine tuning. Design and construction of the focuser affect its precision.

You should be aware that less expensive scopes sacrifice in this area to cut down on costs.

Interchangeable eyepieces

Most spotting scopes are not interchangeable between brands. Also, unlike astronomical scopes, spotting scopes mostly have fixed eyepieces, which the magnification or zoom range cannot be changed. It is important to find one that has interchangeable scope if that feature is desirable, which it is when viewing in the wild.

Durability

To make the scope durable and robust, manufacturers make them waterproof and fog proof. This prevents moisture from penetrating the scope, and fog condensation on the inner surfaces when temperature changes occur. To safely use it on the trail, the body of the scope is also covered with rubber to make it impact proof and to withstand rough use. It is worth noting that mention of these features by the manufacturer is not indicative of its quality. Even the cheapest scopes do claim to have them.

Use

When choosing a scope, you need to determine what you will be using it for and what are the most important features for that kind of use. Then choose the highest quality you can afford based on the features you need most. If I am going to use a scope in a dry environment and will be using a vehicle to transport it, then I won't focus so much on weight, waterproofing and rubber armoring. I will ignore them in favor of a high quality and large aperture, and high magnification.

If I will be hauling it on long treks on a hunting trip, I will go for a lighter scope. It will have a small aperture but will be rubber armored. It should also be easy to mount on my rifle. If I decide to go for whale watching, the water and fog proofing will be a must. Weight, on the other hand, will not be an issue. Alternatively, if am doing aquatic bird watching in marches, the scope should be light weight, fog proof, and waterproof.

Related: Best Spotting Scope for Hunting

For all viewing purposes, I will not compromise on excellent optical properties and mechanical quality of the scope. When comparing scopes, they should be compared logically. I, for example, will not compare a $200 scope with a $1000 scope regardless of what specifications are promised. I can, however, compare scopes from different brands in the same price range. Like a $350 scope from "manufacturer A." to a $400 scope from "manufacturer B."

Testing

To fairly test the scopes, they must be subjected to the same test. I can pin an open newspaper on the wall and test the resolution of the scopes with it. If I want to test for resolution, color fidelity, and color contrast, I would try to view pine needles and small twigs on hillside trees located over 100 yards away. At night, I would use candles shielded in a glass at a distant to test for any distortion and light gathering ability of each scope.

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